* {
    padding: 0;
    margin: 0;
}

body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    background-color: #000;
}

a {
    position: relative;
    width: 400px;
    height: 100px;
    line-height: 100px;
    text-align: center;
    text-decoration: none;
    text-transform: uppercase;
    font-size: 24px;
    color: #fff;
    border-radius: 50px;
    background-image: linear-gradient(to right, #03a9f4, #f441a5, #ffeb3b, #09a8f4);
    background-size: 400%;
    z-index: 1;
}

a::before {
    content: "";
    position: absolute;
    top: -5px;
    bottom: -5px;
    left: -5px;
    right: -5px;
    border-radius: 50px;
    background-image: linear-gradient(to right, #03a9f4, #f441a5, #ffeb3b, #09a8f4);
    background-size: 400%;
    z-index: -1;
    filter: blur(20px);
}

a:hover {
    animation: sun 8s infinite;
}

a:hover::before {
    animation: sun 8s infinite;
}

@keyframes sun {
    100% {
        background-position: -400% 0;
    }
}